Conduct a prospective study in homosexuals with the following specific objectives: observe and study the natural history of the disease in enough persons who are uninfected at the outset to yield a number of cases of AIDS sufficient for meaningful estimates of risk; build a repository as a national resource for specimens and data from men who traversethe entire course from well to ill which would permit testing of hypotheses about etiological factors; and complement similar smaller or less well standardized follow-up studies being performed in different places and times.